I use this as a nickname for the familiar story that tells us that competitive, risk-taking masculinity has evolved for reproductive success, and it’s therefore built into male brains and fuelled by testosterone.

I thought Testosterone Rex was a good nickname for two reasons. ‘Rex’ means king, and this view seems to give an explanation for why men still tend to have more power and wealth than women. And secondly, the set of ideas that Testosterone Rex is based on is now scientifically extinct.

What are the problems with this view?

One problem is that Testosterone Rex is based on an outdated version of evolutionary biology, which assumes that sexual competition is only important for males. This idea came from the observation that because reproduction is cheaper for males than it is for females (in humans, for example, the father can supply just a single sperm, while the mother will provide months of gestation, plus labour and breastfeeding), the risks of competition for status, resources and mates are only worth it for males. But the economics of reproduction turn out to be much more nuanced than this. Sex roles are diverse and dynamic, and a female’s rank and resources can make a big difference to her reproductive success, particularly in mammals.

The T Rex view also assumes that male and female ‘adaptive behaviour’ – ways of behaving that would have increased reproductive success in our evolutionary past – is locked into our sex chromosomes and hormones. But even in other species, these adaptations can disappear or even flip between ‘masculine’ and ‘feminine’ when something relevant in the environment changes. Consider what this means for humans. We inherit a rich culture with norms, values and expectations that can and do change over time, and the environment in which we develop is completely different to our ancestors’. Today, we have contraception, equal opportunity legislation, paternity leave and modern technology, all of which have affected our gendered behaviour.

So how important a role does testosterone play in shaping gender differences?

When we think, “oh, men are like this, women are like that”, testosterone seems like an obvious explanation since males are exposed to much more of it than females. But male-female differences in ‘masculine’ traits like risk-taking and promiscuity are much smaller than differences in testosterone levels, so there isn’t a simple relationship between testosterone level and masculinity. This fits with what we know about testosterone. The levels in the blood are just one part of a very complex hormonal system, and testosterone is just one of many factors that feeds into decision-making and behaviour.

What does this all mean for how we think about gender?

The belief that differences between the sexes are large, fixed and deeply biological is not helpful if we’re going to have a more balanced society, whether that’s more boys playing with dolls, more dads caring for kids, or more women in science and senior leadership roles.

But also, whenever we debate gender equality, in the background is always the idea that natural limits will be set by the fact that males, not females, have evolved to compete for status and resources, and females to care. The science is now showing that the fundamental assumptions behind this are under question – Testosterone Rex is dead, and it’s time to find a successor.